1. Field of the Invention
The present invention relates to a display device driver circuit. A driver circuit is a circuit for converting driving voltages. The driver circuit relating to the present invention is used to drive liquid crystal displays (LCDs), for example.
2. Description of Related Art
A control circuit is used to drive display devices such as LCDs. Such a control circuit is generally constituted of logic integrated circuits. The LCD control circuit applies the driving voltage in the segment direction and driving voltage in the common direction, that is perpendicular to the segment direction, to the liquid crystal elements. Switching the orientations of the driving voltages applied to the liquid crystal elements switches the transparency and opacity to light incident to the liquid crystal element.
The optimum value of the driving voltage VUO applied to the liquid crystal elements varies depending on the type of liquid crystal. In a usual LCD, the optimum value of the driving voltage VUO will be about six volts at the least and about 50 volts at the most. On the other hand, the driving voltage VDD for a usual logic integrated circuit is three to five volts. The output signal voltage of a logic integrated circuit matches the voltage VDD and is therefore three to five volts. Consequently, the output signal voltage of the logic integrated circuit is preferably not applied without further processing to the LCD as the driving voltage VUO.
A usual LCD control circuit generates the voltage to drive the LCD by converting the output signal voltage of a logic circuit from VDD to VUO. The circuit performing this conversion is called a driver circuit. The driver circuit is established at the final stage of the LCD control circuit.
In order for the conversion of the output signal voltage from VDD to VUO, the voltage VUO must be supplied to the driver circuit. The voltage VUO may also be generated by a power source independent from the power source of the voltage VDD, but in many cases is generated by raising the voltage VDD with a voltage booster circuit.
The driving voltage output terminals of the driver circuit are connected to two switching circuits. One switching circuit connects and disconnects the output terminal and the driving voltage VUO. The other switching circuit connects and disconnects the output terminal and ground. When the switch on the voltage VUO side is closed and the switch on the ground side is open, the output terminal outputs the voltage VUO. Oppositely, when the switch on the voltage VUO side is open and the switch on the ground side is closed, the output terminal outputs zero volts. The opening and closing of these switching circuits is controlled by the output signals of the logic circuit in the previous stage.
When the output voltage of the driver circuit is being switched, the timing for closing one switching circuit is faster than the timing for opening the other switching circuit; as a result, a time when both switching circuits are closed will occur. In this case, pass-through current will flow from the voltage VUO side to the ground side. This pass-through current causes the level of the voltage VUO to drop. This level drop causes deterioration of the LCD image quality. When the voltage VUO is generated by the voltage booster circuit discussed above, the level drop of the voltage VUO becomes particularly great. This is because the voltage VUO generated by the voltage booster circuit is greatly dependent on changes to the size of the load.
It is an object of the present invention to provide a driver circuit wherein pass-through current is not generated during the switching of the output voltage.
For this reason, the display device driver circuit relating to the present invention comprises first switching means for switching connection and disconnection between the output terminal and the first source line; second switching means for switching connection and disconnection between the output terminal and the second source line; and selecting means for switching the voltage output from the output terminal by switching the first and second switching means open and closed, so that the timing for opening one switching means is faster than the timing for closing the other switching means.
The selecting means control the first and second switching means so that the timing for opening one switching means is faster than the timing for closing the other switching means. Consequently, a time when both switching means are closed will occur regularly when the output voltage is switched; therefore, no current will pass through these switching means.